Production of resistant silver surfaces



Patented Mar. 29, 1932 UNITED STATES PATENT OFFICE FARBENIN'DUSTRIE AKTIENGESELLS CHAFTi MANY, A. CORPORATION OF GERMANY OF FBANKFORT-ONTHE-MAIN,' GER TRODUOTION 0F RESISTANT SILVER SURFACES No Drawing. Application filed October 29, 1928, Serial No. 815,934, and in Germany November fi, 1927.

rosive action of liquids or vapors containing chlorine, hydrochloric acid, solutions of chlorides and the like has to be considered.

The resistance of silver towards these agents is based substantially on the insolubility of the superficial coating of silver chloride formed. Frequently, as for example in the case of concentrated aqueous hydrochloric acid, this resistance is not great enough to fulfill technical requirements. It has already been suggested to increase the chemical resistance' of silver by alloying it with certain base metals which are absorbed by the silver and form therewith a solid solution. Utensils made of such materials can be employed in many cases.

I have now found that strong resistance to corrosion is obtained, in a very simple and advantageous manner, even in the case of finished articles of silver or of insufficiently resistant alloys rich in silver, by treating the surface which is to be protected,at temperatures above 500 0., but below themelting points of the components, with a metal less electropositive than silver or with an alloy of such a metal, capable of forming with the silver solid solutions which are rich in silver.

As metals which are suitable for the said purpose copper, tin, cadmium, zinc, antimony, aluminum, thallium, magnesium, and in particular manganese and manganese aluminium alloys, may be mentioned. Metals of a low melting point such as tin or antimony are preferably used in the form of al loys of a higher melting point, such as for instance antimony containing 18.2 per cent of aluminium. By means of this treatment the preparation of alloys by melting, prior to the manufacture of the appliances or articles, is rendered superfluous. On the con-- 'trary, the finished silver articles, even when the same have already been used, can be .provided with protective coatings in the specified manner, and thereby provided with an excellent power for resisting corrosion,'as

though the said articles were made'of the silver alloys. Owing to the greater hardness of the solid solutions, the mechanical properties of the said; articles, if made of pure'silver, are also substantially improved.

The. process can be carried out, for example, by treating the surface of the article which it is desired to protect, in an inert atmosphere and at temperatures above 500 C. but below the melting point of the components, with the desired metal which forms solid solutions rich in silver with the silver,

said metal being in the form of powder.

The improvement of the surface area herein described is not restricted to articles of solid silver, but can also be applied to silver plate. silver deposited on the basis metal, such as copper, should preferably be thicker than is usually the case, since the high temperature required for the process also produces diffusion at the margin between the basis metal and the plating, that is to say, reduces the thickness of the latter. The superficial silver alloy produced according to the present invention is inseparably united with the substratum.

The following example will further illustrate the nature of the said invention which however is not limited thereto.

Example In this latter case, the coating of The silver article which is thoroughly cleaned with an abrasive, is embedded in finely powdered manganese or a manganese aluminium alloy, prepared by melting, for ex-- ample by the aluminothermal process and containing, for example, about per cent to be completely surrounded by the powder. The article is then heated to between 600 and 960 centigrade for a considerable time, say for several hours, in an inert atmosphere, the temperature and duration of the heating being dependent on of manganese, so as the desired thickness of the diffusion layer.

By modifying these two factors, it is possible to extend the superficial improvement to any desired depth and to vary the superficial 1 content of manganese. or manganese aluminmm 1n accordance with the required chemical and mechanical resistance. If only parts of the surface of a silver article, such as the inside of a tube, are to be treated, the article need onl be covered by the powder in such parts. he finished work can be scoured with emery or the sandblast, and then polished like fine silver.

What I claim is 1. As'new articles of manufacture, silver articles having a highly resistant surface of an alloy of silver with a metal less electropositive than silver and capable of forming solid solutions with silver, said surface being supported by a less resistant silver-containing material.

2. As new articles of manufacture, silver articles having a highly resistant surfaceofan alloy of silver with manganese, said surfacelbeing supported by a less resistant silver-containing material.

, 3. As new articles of manufacture, silver articles having a highly resistant surface of an alloy of silver with manganese and aluminium, said surface being supported by a less resistant silver-containing material.

In testimony whereof I have hereunto set my hand.

a PETER ASSMANN. 

